There are many different processes for the polymerization of olefins, including gas-phase fluidized bed processes, slurry loop or stirred tank reactors, suspension and solution processes.
For example, it is well known that many polymers can be produced as powders in gas-phase fluid bed reactors, where the fluidization of the polymeric solids is provided by a circulating mixture of gases including one or more monomers. This type of polymerization process is a common process, widely used for the production of polyolefins, such as polyethylene, polypropylene, and polyolefin copolymers. One particular arrangement of a fluid bed polyolefin process is disclosed in U.S. Pat. No. 4,882,400. Other examples of fluid bed polyolefin technology are described in, for example, U.S. Pat. Nos. 3,709,853; 4,003,712; 4,011,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749; 5,541,270; 7,122,607, and 7,300,987. These patents disclose gas-phase polymerization processes where the polymerization medium is either mechanically agitated or fluidized by the continuous flow of gaseous monomer and diluent.
The “traditional” gas-phase fluidized-bed reactors described in many of the patents listed above is a simple and cost-competitive device useful for the manufacture of polyolefins. One example of a prior art polymerization process is illustrated in FIG. 1 (prior art). A catalyst is fed through supply line 2 into a fluidized bed reactor 4 and, simultaneously, a gaseous olefin is caused to pass through recycle line 6 and dispersed into the bottom of the fluidized bed reactor 4 through a gas distributor plate 8. The gas distributor plate 8 may include, for example, a perforated plate having a plurality of through holes, and is arranged in the vicinity of the bottom of the fluidized bed reactor 4. In this way, a fluidized bed 10 is formed and held in the fluidized state in the fluidized bed reactor 4. Polymerization of the monomer is carried out in the fluidized bed 10, and polymer particles produced by the polymerization reaction are continuously discharged through line 12. Unreacted gaseous olefin having passed through the fluidized bed 10 has its flow rate reduced in a velocity reduction zone 14 provided in an upper part of the fluidized bed reactor 4, where the vapor velocity is reduced so as to avoid or reduce entrainment of polymer particles from fluidized bed 10. The unreacted monomer is discharged outside the fluidized bed reactor 4 through gas outlet 16 to gas recirculation line 18 disposed at a top of velocity reduction zone 14. The unreacted gaseous olefin is then recycled via line 18 to the bottom of fluidized bed reactor 4 via compressor 20. Monomer added via line 24 accounts for monomer reacted to form the polymer removed via flow line 12 together with a small amount of dissolved monomer, thus maintaining a constant supply of monomer to reactor 4. The heat of polymerization generated in fluid bed 10 may be removed from the system by cooling the recycle gas in heat exchanger 22, which may be located upstream or downstream of compressor 20.
The active, growing powder in a fluidized bed polyolefin reactor, such as that described in FIG. 1 and the aforementioned patents, contains a wide range of particle sizes. Thus, the powder is referred to as having a broad particle size distribution. Some of the reasons for the broad size distribution are the size range of the initial catalyst particles (or prepolymer particles) charged to the reactor, the difference in catalytic activity of each catalyst particle, the difference in residence time for each growing polymer particle, the agglomeration of polymer particles, and the spalling of polymer particles.
While the above-described gas-phase reactor is extremely useful for performing polymerization reactions, many desired polyolefin products are difficult to produce in such gas-phase reactors, including bimodal and multimodal products or products having broad molecular weight distributions and other advanced products, and materials utilizing comonomers that have a higher boiling point that are normally liquids at elevated temperatures and pressures. Such products typically require the use of specialized catalysts, such as dual site or bimodal catalysts or the use of multiple reactors in series. Polymerization with such specialized catalysts often results in reactor operability issues. Additionally, traditional gas-phase reactors include various limitations on heat removal (production rate capacity), transition time between different products and catalysts, catalyst temperature rise and heat removal from a catalyst particle, and particle agglomeration due to reactor static and/or insufficient heat removal from particles resulting in softening of the produced polymer.
One attempt to overcome the deficiencies in a traditional gas-phase reactor is described in U.S. Pat. No. 5,698,642 disclosing a multi-zone circulating reactor (MZCR) in which there is an up-flowing riser section operating in a dilute-phase fast fluidization regime and a down-flowing dense-phase moving-bed section. The gas compositions in those two sections are set differently to achieve the product differentiation. WO 2006/022736 discloses a reactor system composing a plurality of MZCRs connected in fluid communication, and describes different types of operation for the different reactor zones.
With respect to the MZCR, the dense phase down-flowing moving bed may be agglomeration prone and may cause significant problem in reactor operation. Pre-polymerization is required for the MZCR, although it can not solve all the agglomeration-related reactor-operation problems. For example, see P. Cai, I. D. Burdett, “Polymerization Simulation Under Different Fluidization Regimes,” Circulating Fluidized Bed Technology VIII, ed. by K. Cen, p. 410-417, International Academic Publishers (2005). In addition, it is very difficult to control the temperature uniformity in the down-flowing moving bed, which in turn can result in a negative impact on product property control.
Additional limitations of traditional gas-phase reactors can include limitations on reactor operability, range of polymers that may be produced, turndown capacity, particle sintering and throughput (polymer production rate), among other limitations.